1. Field of the Invention
The invention relates in general to a method of manufacturing a polysilicon layer, and more particularly to a method of manufacturing a polysilicon layer using a mask for the laser beam to preheat an amorphous silicon layer and completely melt the preheated amorphous silicon layer, and the mask therein.
2. Description of the Related Art
With the rapid development in science and technology, display panel has been widely used in electronic devices such as notebook, personal digital assistant (PDA) and mobile phone. The display panel has two categories, namely, the amorphous silicon (a-Si) thin film transistor (TFT) display panel and the low temperature polysilicon (LTPS) TFT display panel. The main difference between the LTPS TFT display panel and the amorphous silicon TFT display panel is that the LTPS TFT display panel converts an amorphous silicon layer into a polysilicon layer via laser annealing, largely enhancing the electron mobility of the TFT. The panel driving circuit and the integrated circuit (IC) can be integrated into the LTPS TFT display panel without having extra circuit board design, largely increasing the flexibility in the design of panel and circuit. Therefore, the LTPS TFT display panel is a display panel with great potential.
Referring to FIGS. 1A˜1C, flowchart diagrams of a conventional method of manufacturing a polysilicon layer are shown. Firstly, the method begins at FIG. 1A: a substrate is provided 11 and an amorphous silicon layer is formed 12 on the substrate 11. Next, a mask 15 having a non-transparent region 15a and a transparent region 15c is provided atop of the amorphous silicon layer 12, and a laser beam 16 is projected onto the mask 15. Next, as shown in FIG. 1B, the laser beam 16 penetrates through the transparent region 15c and completely melts part of the amorphous silicon layer 12 for the melted amorphous silicon layer 12b and the remaining amorphous silicon layer 12a to be formed on the substrate 11. Since temperature gradient exists between the remaining amorphous silicon layer 12a and the melted amorphous silicon layer 12b, the temperature of the melted amorphous silicon layer 12b is higher than the temperature of the remaining amorphous silicon layer 12a, enabling the melted amorphous silicon layer 12b to use the remaining amorphous silicon layer 12a as seed crystal to laterally form crystallization and become a polysilicon layer 13 along the direction of the arrows 20a and 20b as shown in FIG. 1C. The technology, which uses a laser beam 16 to penetrate through a mask 15 having a non-transparent region 15a and a transparent region 15c to completely melt part of the polysilicon layer 12 for the polysilicon layer 12 to form crystallization laterally, is called the sequence lateral solidification (SLS) method or the excimer laser annealed lateral crystallization (ELA-LC) method. In FIG. 1C, the polysilicon layer 13 has a crystal lattice or boundary 13b. Furthermore, the polysilicon layer 13 corresponds to a defect 13a projected from the surface of the crystal lattice or boundary 13b, affecting the adhesiveness between the insulation layer and the polysilicon layer 13 afterwards.
The distance of lateral crystallization can be enlarged by prolonging laser pulse duration time or by heating the substrate 11 to prolong the silicon melting time. However, according to conventional practice, the manufacturing facility must have a substrate heater to heat the overall piece of the substrate 11. In order to have optimum energy for the temperature gradient and the laser beam 16 to melt the amorphous silicon layer 12, the method of heating the overall piece of the substrate 11 would increase the optimum energy, largely shortening the lifespan and the maintenance period of the laser beam optical module. Besides, the polysilicon layer 13 formed according to the laser annealing method have a large number of defects 13a existing on the crystal lattice or boundary 13b. Although the defects 13a can be repaired by using the laser beam 16 again or using high-temperature treatment, additional manufacturing steps and manufacturing costs would occur. Besides, only part of the laser beam 16 would penetrate through the transparent region 15c and melt part of the amorphous silicon layer 12, so another part of the laser beam 16 would be partly reflected and absorbed by the non-transparent region 15a and converted into heat, resulting in a poor utilization rate of the laser beam 16.